There is a continuing need in the printed circuit, graphic arts, and other related industries, to transfer images photographically from a sequence of original (positive or negative) photomasks to one or both sides of a light sensitive sheet support. There is a particular need to prepare four-color surprint proofs from original artwork using suitable photomasks. In making the surprint proof, for example, it is very important to align each photomask with the preceding image so that the final image is precisely in register with the preceding image. There is also a need to make multilayer circuit boards on one or both sides of a support also using a multitude of photomasks. In the case where multilayered printed circuit boards are being made with multiple images on one or both sides of the board, it is also important to precisely register these images with each other. In all instances, the photographic process must be carried out in a manner which maintains the definition, location and image features of the original and minimizes the image dislocation. Finally, it is important to automate as much of this process as possible in order to reduce operator handling and improve reproducibility.
Four-color surprint proofs can be prepared as taught for example in U.S. Pat. No. 3,649,268 and U.S. Pat. No. 3,854,950 (positive working process) and U.S. Pat. No. 4,174,216 (negative working process). In these processes, a photopolymer film element is first laminated to a suitable support such as paper card stock. This film is then given an imagewise exposure to actinic light through one of four photomasks (e.g. color separation films representing each of the four colors from the original artwork) which bears one of the primary colors. Normally, a vacuum frame device is used during the exposure process to insure good photomask/photosensitive layer contact. In the positive working proces, the photopolymer film is inherently tacky and those areas struck by light during exposure, harden and become less tacky. In the negative working process, the photopolymer film is associated with a tacky, adherent layer and exposure causes the photopolymer layer to fracture imagewise. When the cover layer associated with this structure is then peeled off, the exposed areas relative to this process are removed uncovering imagewise areas of the tacky adherent layer. The remaining, unexposed areas are not tacky. In both of these processes, the final image is developed by applying a colored, powdered toner to the tacky areas. The color of the toner should correspond to the color recorded in the photomask image. The toner adheres to the tacky portion of the image revealing the copy. For each additional color record, an additional layer of photopolymer is laminated over the preceding image and exposed to its photomask. Each exposure must be made in precise registry with the preceding exposure in order to maintain the correct image location. Typically, a set of four photomask image records using colored, powdered toners of yellow, magenta, cyan and black are used to prepare a final image in this sequential operation. This process, then, will produce an exact surprint proof of the original artwork and is an excellent method for proofing originals for the printing industry. Each colored toner can be matched closely to the desired printing ink and thus the process is very useful. It is desirable to automate the surprint making process since the need to expose each photomask in a vacuum frame apparatus is time consuming. Additionally, registration of each record is difficult to do by hand and even more difficult to automate.
Multilayer printed circuit boards can also be prepared by a sequential image transfer process using a dry photopolymer film and an additive plating process such as that described in U.S. Pat. Nos. 4,054,483; 4,054,479; and 4,157,407. In the general processes described in these patents, a photopolymer film element is laminated to a sheet substrate material (e.g., a thin insulated board) and the photopolymer layer is exposed imagewise to actinic radiation through a photomask bearing a printed circuit image. When the sheet substrate contains circuit components such as through-holes or a circuit conductor pattern, the photomask must first be registered to the component before the exposure step. Either the imaging exposure itself, or subsequent process steps will produce adherent image areas on the laminated substrate into which powdered catalyst can be suitably imbedded. The powdered catalyst (e.g., copper powder) is then applied to the imaged surface to produce a catalytic circuit pattern which is then plated (e.g., with an electroless plating bath). These process steps will produce a highly conductive circuit pattern from the original artwork. For each additional layer of circuitry needed to complete the multilayered circuit, a fresh photopolymer film is laminated to the surface of the previously imaged and plated laminate, exposed again imagewise to its photomask which is held in precise registry with the previous image, and the entire process outlined above repeated. Using this process, multiple circuit layers can be applied to one or both sides of a substrate and circuit layer interconnections and through-holes can be formed.
Typically, manual transfer and positioning of the substrate element has occurred between each step of the process described above. The steps of application of the catalyst are also carried out by hand. Attempts have been made to automate these processes but this has been adopted only to a limited extent such as the application of the catalyst. Automation will result in a substantial cost savings to the user and is a highly desirable feature. Nevertheless, many of the above-described steps of this process remain labor intensive and prone to human error. This is particularly true of the registration and exposure steps.
Contact printing is virtually the universal method of exposure used today in the surprint proof and printed circuit photofabrication industries despite certain known shortcomings. Although low in equipment costs, simple to use, and capable of excellent line and halftone definition, contact printing is labor intensive and slow because of the previously mentioned vacuum drawdown times. It is also subject to losses due to damaged and dirty photomasks resulting from repeated use. This, in turn, requires frequent and expensive touch up and replacement of expensive photomasks to avoid yield losses. Much time is also lost in the constant and tedious process of inspecting the photomasks for defects between exposures. In addition, variations in frame temperature and ambient humidity affect corner-to-corner registration, especially when using larger formats.
Alternative exposure methods such as gap printing, projection printing and laser scanning each offer some significant advantages over contact printing. However, in the current state of development, all have serious limitations for high productivity applications and are intrinsically higher in equipment costs.